These advanced steps are primarily for system administrators and others who are familiar with the command line. You don't need a bootable installer to upgrade macOS or reinstall macOS, but it can be useful when you want to install on multiple computers without downloading the installer each time.
What you need to create a bootable installer
- A USB flash drive or other secondary volume formatted as Mac OS Extended, with at least 14GB of available storage
- A downloaded installer for macOS Big Sur, Catalina, Mojave, High Sierra, or El Capitan
To install Catalina on another Mac, first make sure it's powered off, then simply plug in the USB installer you created and turn the Mac on. When you hear the boot tone, hold down the Option. Select the.dmg file with the macOS installation files. Click the Open button. Click the OK button. Click the Yes button. Once you complete the steps, you can now insert the USB flash drive on your Mac computer to install, reinstall, or upgrade the operating system to the latest macOS version, which can be Sierra, Catalina, Big Sur, or higher.
Download macOS
- Download: macOS Big Sur, macOS Catalina, macOS Mojave, or macOS High Sierra
These download to your Applications folder as an app named Install macOS [version name]. If the installer opens after downloading, quit it without continuing installation. To get the correct installer, download from a Mac that is using macOS Sierra 10.12.5 or later, or El Capitan 10.11.6. Enterprise administrators, please download from Apple, not a locally hosted software-update server. - Download: OS X El Capitan
This downloads as a disk image named InstallMacOSX.dmg. On a Mac that is compatible with El Capitan, open the disk image and run the installer within, named InstallMacOSX.pkg. It installs an app named Install OS X El Capitan into your Applications folder. You will create the bootable installer from this app, not from the disk image or .pkg installer.
Use the 'createinstallmedia' command in Terminal
- Connect the USB flash drive or other volume that you're using for the bootable installer.
- Open Terminal, which is in the Utilities folder of your Applications folder.
- Type or paste one of the following commands in Terminal. These assume that the installer is in your Applications folder, and MyVolume is the name of the USB flash drive or other volume you're using. If it has a different name, replace
MyVolumein these commands with the name of your volume.
Big Sur:*
Catalina:*
Mojave:*
High Sierra:*
El Capitan:
* If your Mac is using macOS Sierra or earlier, include the --applicationpath argument and installer path, similar to the way this is done in the command for El Capitan.
After typing the command:
- Press Return to enter the command.
- When prompted, type your administrator password and press Return again. Terminal doesn't show any characters as you type your password.
- When prompted, type
Yto confirm that you want to erase the volume, then press Return. Terminal shows the progress as the volume is erased. - After the volume is erased, you may see an alert that Terminal would like to access files on a removable volume. Click OK to allow the copy to proceed.
- When Terminal says that it's done, the volume will have the same name as the installer you downloaded, such as Install macOS Big Sur. You can now quit Terminal and eject the volume.
Use the bootable installer
Determine whether you're using a Mac with Apple silicon, then follow the appropriate steps:
Apple silicon
- Plug the bootable installer into a Mac that is connected to the internet and compatible with the version of macOS you're installing.
- Turn on your Mac and continue to hold the power button until you see the startup options window, which shows your bootable volumes.
- Select the volume containing the bootable installer, then click Continue.
- When the macOS installer opens, follow the onscreen instructions.
Intel processor
- Plug the bootable installer into a Mac that is connected to the internet and compatible with the version of macOS you're installing.
- Press and hold the Option (Alt) ⌥ key immediately after turning on or restarting your Mac.
- Release the Option key when you see a dark screen showing your bootable volumes.
- Select the volume containing the bootable installer. Then click the up arrow or press Return.
If you can't start up from the bootable installer, make sure that the External Boot setting in Startup Security Utility is set to allow booting from external media. - Choose your language, if prompted.
- Select Install macOS (or Install OS X) from the Utilities window, then click Continue and follow the onscreen instructions.
Learn more
A bootable installer doesn't download macOS from the internet, but it does require an internet connection to get firmware and other information specific to the Mac model.
For information about the createinstallmedia command and the arguments you can use with it, make sure that the macOS installer is in your Applications folder, then enter the appropriate path in Terminal:
Easy way to Clean Install mac OS Catalina
macOS Catalina is Apple’s shiny new operating system for the Mac and it’s got a ton of great features and improvements. You’re probably keen to get started and install it straight away. Before you do, consider whether you want to install it over the top of your current OS, or as a completely clean install of macOS Catalina. The benefits of the latter are that it will be like a completely new Mac, with all your settings wiped out, and junk files gone. It should make your Mac run faster. The rest of this article will explain how to clean install macOS Catalina.
Before the clean installation of macOS 10.15 Catalina it’s recommended that you first delete junk and dusty files from the current macOS. There are many free and paid tools to clean junk but the quickest option is to run CleanMyMac X. You can expect to free up tens of gigabytes of space, including old apps, and their leftovers.
Here’s how it works.
- Download and install CleanMyMac — try a free edition of the app here
- Choose the System Junk tool.
- Press Scan.
- When it’s finished either press Remove to get rid of all the files it recommends, or choose Review to look through what it’s found and decide for yourself what to delete and what to keep.
Backup your Mac
I can’t stress how important this is. You must backup your Mac before you do anything else. You’re going to be erasing your startup drive completely, so you need a complete backup of everything. You can use Time Machine, another backup app, or a cloning tool. Just make sure you have a complete, preferably bootable, copy of your current installation.
You should also make notes of settings for apps you use regularly – screen grabbing them is a good way to do that. And you should make sure you have a copy of any licence codes that you’ll need when you reinstall applications. You can use Apple’s Notes app to make notes, as long as you sync them with iCloud.
Create a bootable installer of macOS Catalina
If you’re reading this after Catalina ships, you can download it from the Mac App Store and then choose Quit when the installation starts. If you’re clean installing the beta (and, by the way, that’s not something I would recommend. It’s much better to create a dual boot system when you run the beta.
- Grab a USB stick with a capacity of at least 16GB, or an external hard drive or USB.
- Go to Applications>Utilities and launch Disk Utility.
- Launch Disk Utilities and Erase the Drive or USB stick, formatting it as Mac OS Extended (Journaled) or APFS and selecting GUID partition map if a selection is available.
- The easiest method for creating a bootable installer is to use DiskMaker X. However, if you’re comfortable using Terminal, you can do it like this:
- Launch Terminal from Applications>Utilities
- Type:
sudo /Applications/Install macOS Catalina Beta.app/Contents/Resources/createinstallmedia --volume /Volumes/Untitled --applicationpath /Applications/Install macOS Catalina Beta.appwhere ‘Untitled’ is the name of your USB stick or external hard drive - Press Return and wait for the word Done to appear in Terminal.
Install Catalina Via Usb
Once you see ‘Done’ in Terminal you can quit it. Your USB stick or external hard drive is now a bootable Catalina installer, so keep it safe. Now, it’s time to clean install macOS Catalina.
How to Clean install macOS Catalina
Now you've made a backup and created a bootable installer, it's time to actually install the new macOS. Follow these easy steps to clean install macOS Catalina on your Mac.
Install Catalina From Bootable Usb
- Plug your USB stick or external hard drive into your Mac, if it’s not plugged in already.
- Restart your Mac while holding down the Option key.
- When you see the options for booting appear on screen, choose the installer disk you just created.
- When you see the macOS Utilities screen appear, choose Disk Utility
- When Disk Utility launches, choose the Eras tab then select your startup volume (probably called Macintosh HD).
- Give your startup disk a name, then choose a format. If you have a recent Mac with an SSD startup disk, select APFS. If you have an older Mac with a mechanical hard drive or Fusion Drive, choose whichever one you want to use.
- Choose Erase.
- When Disk Utility has finished erasing the disk, quit it to return to the macOS Utilities screen.
- Choose Install macOS.
- Press Continue and choose the drive you just erased and re-formatted as the destination.
- MacOS Catalina will start to install.
- When it’s finished, your Mac will restart and it will look like a brand new Mac. Go through the set-up process to set up a user account, add your wifi network, choose a language, and log in to iCloud.
Once you’ve set up your Mac, you can eject the installer disk and remove it. Remember to keep it safe. You can now restore files you backed up earlier from Time Machine or whichever backup tool you used. Remember to restore files and folders individually, don’t, for example, restore your entire user directory otherwise you’ll copy back all the junk files you just got rid of. Reinstall applications and go through them adding license details where necessary. Check that everything works as you expect it to and use the screen shots you made earlier to configure settings.
Install Catalina From Usb
There are lots of good reasons why you might want to clean install macOS Catalina, but the most important one is that it should make your Mac feel like new and keep it running smoothly. For ultimate Mac refresh you can also download CleanMyMac X and play with its cleanup tools. Getting rid of old junk feels great.